An extrusion-based additive manufacturing system is described in WO2012/037329. The system uses a filament as consumable feedstock. The filament has a core portion and a shell portion with different peak crystallization temperatures. Both the core and the shell portions are melted in an extrusion head, and after they have been deposited the portion with the higher crystallization temperature crystallizes before the other portion. This allows the extruded material to resist gravity and the pressure of subsequent layers, while also reducing distortions.
The melting temperatures of the core and shell materials are desirably the same or similar to allow the filament to be readily melted in the extrusion head.
The base polymers for the core and shell are each capable of achieving an average percent crystallinity in a solid state of at least 10% by weight. In one embodiment the base polymers for the core and shell are each capable of achieving an average percent crystallinity in a solid state of at least 25% by weight. In another embodiment the base polymers for the core and shell are each capable of achieving an average percent crystallinity in a solid state of at least 50% by weight. The percent crystallinity of the core and shell materials in the filament is not mentioned either in absolute or relative terms, and is presumably not important since both the core and the shell melt before they are extruded. The shell material exhibits at least about 50% crystallinity after deposition and prior to deposition of a subsequent layer.
In an alternative embodiment a double shell is provided in which the inner shell includes a material that restricts or prevents interdiffusion and/or crystal growth between the materials of the core and outer shell. In this alternative embodiment, suitable materials for the inner shell may include one or more amorphous polymers. The melting temperature of the inner shell is not described.